So let’s start with some of the things that are obvious to anyone who works in music but possibly not to others.
If you’re a musician, then playing / singing music is your job. Or, for a lot of us, your line of self-employment – important because the self-employed don’t get sick leave. So once you’re contracted in, you’re going to want to do something – pulling out not only loses you that job but might lose you any future opportunity of working for that client (yes, really. Maybe not thought through quite that explicitly, but really). If you can keep going an illness, whether it’s a cold, a bad back or a particularly bad flare-up of your clinical anxiety, then you will. That’s what it’s like in an overcrowded freelance market – the potential financial consequences of doing otherwise are too severe.
Slightly more unexpected: If you’re a classical musician, or a hired hand, or indeed a band member who’s not your band’s main writer, then your task is not to be a self-expressive artist. If you’re playing second-desk viola in a Mozart symphony, the only feelings you’re going to be creating are those of Mozart when writing it (perhaps) or the conductor when considering their interpretation (also perhaps – both might in fact just be the emotions the relevant people chose to try and convey and they might not at all have been their own at the time. If you’re playing someone else’s song, then you have to fit with it, or at least with the arrangement you may or may not have been involved in thrashing out. And once a track is well-known, then you have to reproduce it in essentials, including mood, each time you perform it. You’re unlikely to feel the same way performing a given set halfway through a six-month tour as you did playing most of those songs ungigged in the recording studio. But you can’t rehash them to suit your emotions at the time because that won’t fit with the expectations of either your fellow performers or your audiences and, oh look, there went your commercial (and quite possibly critical) viability.
So from the above, it would appear that the craft of music is in part involved in placing immediate emotion on one side and performing music as the music is, rather than you are – a sort of sublimated acting. Arguably, a (professional) violinist who is in love, depressed, elated by his football team’s victory, etc., should not play the same piece any differently under those different circumstances, any more than an accountant should analyse the same columns of figures differently when elated, depressed, in love.
But, there may well be a difference to the rate and accuracy of the accountant’s work if they are emotionally or mentally disordered; their teamwork and interlocking with colleagues working on different areas may also benefit or suffer. And music is similar except that (barring in some senses very high-budget recordings) it is carried out in a very immediate and real-time fashion. Ensemble music-making is also a closeness of teamwork rarely found in desk jobs. The result is that someone who is depressed (or just tired, or had an unexpectedly difficult journey to the rehearsal) may take more plays through a passage to nail it than they usually would. If they don’t get the extra plays due to lack of rehearsal time or a set schedule and organisation that prevents extending practice time for the benefit of one individual, then they will be more likely to have not yet reached mastery of the section by the performance, which is when it matters. In a small band, they may also be socially saturnine, prickly or unpredictable, which distracts their bandmates’ attention and makes it more difficult to follow and interact with their musical intentions – quite regardless of whether anyone gets upset by their behaviour.
Even if you take a really quite severely craftspersonlike view of the business of performing music – and when expecting somebody to pay I generally do – it must be acknowledged that extremes of emotion and mental health problems affect, not the ‘expressive style’ of one’s music-making, but rather the dexterity and technique of it; in a not dissimilar way to physical ailments or injuries. In a more distinctive way, they also have the potential to impair ensemble cohesion.
Thus: music does not have to reflect the emotional states of its players / singers. But those emotional states may significantly impact the quality of the music produced.